The mayors of 21 New Jersey cities are demanding action over mysterious drones that have flown overhead in recent weeks in a letter to Gov. Phil Murphy.

Since mid-November, large drones of uncertain origin have been repeatedly spotted in the skies over central and northern New Jersey at night.

The drones, which are larger than the type typically used by hobbyists, have raised concerns because of their proximity to a military installation and President-elect Donald Trump’s Bedminster golf course.

In the letter, Morris County mayors called for a comprehensive investigation into the drones “to determine their origin, purpose and compliance with applicable regulations.”

They also called for “implementation of measures to prevent future unauthorized or unsafe drone operations” in Morris County and “clear communication with local officials, law enforcement and residents about the results of this investigation.”

“While the benefits of drone technology when used responsibly and transparently are widely recognized, the lack of information and clarity about these operations has caused fear and frustration among our constituents,” the mayors wrote. “Despite inquiries to the relevant authorities, we have not yet received satisfactory answers regarding the purpose, operators or safety protocols of these flights.”

The Federal Aviation Administration has imposed flight restrictions on drones while authorities investigate.

Local police previously said there was no known threat to public safety.

“Morris County Sheriff James M. Gannon would like to inform everyone that the recent drone activity that many have observed in our communities is being actively investigated. There is no apparent immediate danger to the public at this time,” the Morris County Sheriff’s Office said in a statement.

Last week, several Morris and Somerset County officials said in a joint statement that they would continue to monitor drone activity.

“County and local law enforcement are aware of recurring reports of drone activity in Morris and Somerset counties and recognize the public’s concern about these repeated sightings,” they said.

On Monday, an FBI spokesman said the agency was “doing everything we can to figure out what’s going on.”

“The public can still call our 800 hotline or submit a tip online. We respond to every material tip we receive,” the FBI spokesman said.

In an interview on Fox News on Monday, New York Rep. Nicole Malliotakis said the public “deserves and demands answers” about drones.

“I have also reached out to the FAA, Department of Homeland Security and FBI for answers and they are aware of these aircraft. They’re either unmanned aircraft or drones, and they’re still trying to figure out where they are.” But after what happened with the Chinese spy balloon, that’s just not enough.

ABC News’ Alex Faul contributed to this report.